The present invention is related to the field of pressure sensors, and more particularly to the field of differential pressure sensors which comprise gauge pressure sensors wherein an unknown pressure is measured with respect to sensed ambient (atmospheric) pressure.
Various types of pressure sensors exist due to the use of different physical effects, such as variations in resistance and/or capacitance which occur in response to sensed pressure changes, can be used for the pressure transducer element. Here we are describing one particular gauge pressure sensor design which developed from a capacitive absolute pressure sensor. These capacitive absolute pressure sensors comprise a pressure transducer element having a displaceable diaphragm wherein a sensed pressure causes corresponding displacements of the diaphragm, which in turn either controls the position of an indicating gauge or controls corresponding electrical characteristics of the sensor that are utilized by subsequent circuitry to provide electrical signals indicative of the sensed pressure. Examples of such pressure sensors which utilize absolute pressure transducer elements, rather than gauge transducer elements, are illustrated in issued U.S. Pat. Nos. 4,178,621 and 4,380,041, both of these references being assigned to the same assignee as the present invention.
It should be noted that the term "pressure transducer" as utilized herein refers to a pressure sensing element which provides various corresponding electrical or mechanical characteristics in response to sensed pressure, whereas the term "pressure sensor" is utilized to refer to an assembly comprising the pressure sensing element itself along with the protective housing in which the sensing element is mounted, and the term may also include various electrical circuitry for converting changes in electrical characteristics to corresponding changes in the electrical characteristics, such as amplitude and/or frequency, of a corresponding pressure related electrical signal.
In absolute pressure transducers, a sensed unknown pressure is applied to one side of a pressure sensing diaphragm while a fixed predetermined reference pressure is applied to the other side of the diaphragm. Thus the displacement of the diaphragm is related to the magnitude of the sensed pressure with respect to the fixed reference pressure. For gauge pressure sensors, the sensed pressure is not compared with respect to a fixed reference pressure but is compared with respect to atmospheric pressure which will correspond to the ambient pressure surrounding the housing of the pressure sensor. For some prior absolute pressure sensors, capacitive pressure transducers have been utilized and examples of these are illustrated in the two references previously noted.
While gauge pressure sensors have existed prior to the present invention, typically their mechanical configuration has not readily permitted their use with capacitive pressure transducer elements while sufficiently guarding against the effect of contaminants in the ambient atmospheric pressure affecting the accuracy of the pressure sensing. This type of deficiency would appear to apply to any type of pressure transducer wherein pressure changes cause changes in electrical characteristics of a monitoring pressure transducer element and wherein atmospheric contaminants can affect these electrical characteristics. In addition, typical prior art gauge pressure transducers could be overly susceptible to unidirectional pressure disturbances that occurred at localized areas external to the pressure sensor housing.